Papermaking apparatus commonly includes a source of aqueous stock containing fibrous solids, a headbox to which the aqueous stock is supplied and a continuous wire belt upon which the headbox discharges a controlled amount of the aqueous stock which is at least partially dewatered by draining through the belt. In addition to the foregoing, there is ordinarily a press which squeezes the water out of the wet paper sheet and finally steam heated rolls which dry and consolidate the paper.
As the uncalendered sheet is calendered and passes through the high temperature/high pressure calender workroll the sheets of certain paper precursors may shrink significantly in the cross-machine direction. The shrinkage increases as the edge of the sheet is approached and a disproportionately high basis weight and thickness at the edges of the calendered sheet may result.
It is also common in papermaking equipment to find two blades defining the discharge opening of the headbox. One, referred to herein as the main blade, extends along the width of the headbox and is vertically moveable to control the size of the discharge opening. The second or auxiliary blade is a straight-edged blade moveably attached to the main blade. It moves with the main blade but is separately adjustable. The auxiliary blade is generally made of steel and can be flexed slightly by adjustment of a plurality of spaced vertical rods, each with screw and worm gear which provide vertical deflection at the point where the rod is secured to the blade. By raising or lowering the main blade one can increase or decrease the velocity at which the stock leaves the headbox. The deflection of the auxiliary blade to accommodate the desired basis weight profile, particularly at the outermost positions frequently causes permanent deformation of the auxiliary blade as its elastic limit is exceeded. Fatigue problems in the metal may also result. This is discussed in Fourdrinier Papermaking , a monograph by G. Gavelin 1963 Lockwood Trade Journal, Inc. and TAPPI Noles 1986 Wet End Operations Seminar. The present invention offers a solution to the problem.